1. Technical Field
The present invention relates to an ester for refrigerator oils, and a method for producing the same. According to the present invention, it is possible to improve thermal stability of the esters used for refrigerator oils.
2. Background Art
Due to concerns over ozone layer depletion, there has been an attempt to shift from chlorine-containing fluorocarbon refrigerants (chlorine-containing fluorocarbon refrigerants) that have been conventionally used in air-conditioning equipment such as air conditioners, and refrigerators, to fluorocarbon refrigerants containing no chlorine (CFC (chlorofluorocarbon) substitute refrigerants). Lubricants that have been conventionally used as refrigerator oils for chlorine-containing fluorocarbon refrigerants, for example, lubricants with mineral oils or synthetic hydrocarbon oils such as alkylbenzene, have a low compatibility with CFC substitute refrigerants, so that it has become difficult for the conventional lubricants to achieve its full capacity as refrigerator oils due to the shift in refrigerants.
In order to solve the above-noted problem, there has been a study on compounds that show good compatibility with CFC substitute refrigerants. As a result, for example, polyol ester compounds and polyalkylene glycol compounds were found as refrigeration oils, and they have begun to be used instead of conventional refrigerator oils using mineral oils or synthetic hydrocarbon oils.
Among them, a polyol ester is an ester of a neopentyl polyol, such as neopentyl glycol, trimethylol propane, pentaerythritol and dipentaerythritol, and an aliphatic carboxylic acid, and it has been widely used because it has not only an excellent compatibility with CFC substitute refrigerants but also its electrical insulating properties and thermal stability which are suitable for a refrigerator oil.
In recent years, as equipment using the above refrigerator oils has become more compact (use of less refrigerator oil per unit) and saving energy (extension of operation time of a compressor due to inverter control) has been achieved, the working conditions of a refrigerator oil have become more demanding and high quality is required more than ever before for a refrigerator oil. For example, there is a possibility that refrigerator oil is thermally decomposed as it is locally exposed to a high temperature condition with frictional heat at a sliding portion of a compressor, and the generated decomposition product corrodes metal members and causes an adverse effect on the resin material.
Based on the above, refrigerator oil has been required to have a higher thermal stability than ever before.
It is known that the thermal stability of polyol esters relies not only on the chemical structure of polyol esters but also on the production method. Therefore, various methods for obtaining an ester for refrigerator oils having high thermal stability have ever been reported.
For example, Patent Literature 1 discloses a method for simultaneously adding a Lewis acid catalyst and a phosphorus-based reducing agent in an esterification reaction as a production method for obtaining an ester having a high thermal stability.
Also, Patent Literature 2 discloses a method of neutralization with an alkali aqueous solution after adding a hydrocarbon solvent and an alcohol solvent as a method for purifying an ester in the production of an ester having a high thermal stability.
Patent Literature 3 discloses a method for producing an ester for refrigerator oils including a step of adsorbing unreacted carboxylic acid with an inorganic acid-acceptor and then removing the remaining carboxylate in ester by using activated carbon, etc. as a method for producing an ester having excellent stability over a long period.